Ah! Thanks. I've run Mint at various times over the years and never had this happen before, so I assumed it was a new thing with the Debian version. Scanned the headlines and didn't see it mentioned, and didn't think to search. Sorry about that.

In case someone else finds this while searching for the same thing, the fix is here (an additional link away from the post that you linked to, BTW):

The problem is when you click on the Package Manager the menu stays open. This causes the error because there is a security function to disallow keyloggers to trap your password.

There are 2 ways to fix this:

1. After clicking on the package manager, click your desktop, this closes the menu.or2. Make sure all Items are showing in the menu (Place,Applications,System). There should be no "tab" bar on the left and a blank dot at the top. When you click on any item in the menu now it should close the menu.